When you or your recipients sees an "Authorization Required" page from Salesforce, this typically means that there is a permissions error in play. Unfortunately, Salesforce does not provide a way to easily interpret which permission is causing the error, therefore it's not possible to provide more details to the visitor or to your organization as to why the error is caused at this time.
As Salesforce's error reporting for permission errors becomes more verbose, we will investigate ways to provide these details to your organization as well.
In the meantime, here are a few troubleshooting articles that can help you debug these situations.
1) Why do I see an "Authorization Required" page when I press the Preview button on a campaign?